It is known to treat surfaces of steel, aluminum, and/or their alloys, to remove therefrom unsightly and corrosion-promoting oxide layers, such as are formed by contact with the environment and/or in the course of welding or other heat treatments of the metal surfaces. Typically, this is done by treatment with acid solutions, such as those based on hydrochloric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, and the like.
In some cases, particularly with large parts, where there are localized areas of oxide formation, acid-containing paste have been applied only to the places that are to be cleaned. In this manner, part of the oxides and the underlying metal attached to them are dissolved, so that the oxides lose their adhesion to the metal surface. After an appropriate contact time with the paste composition, it can be rinsed off with water or removed by brushing.
Typical of pickling and rust removing paste of this type are those based on phosphoric acid and containing, as a thickener, a saponifiable oil or a fatty acid which can be converted to a soap. Such compositions are described in West German Auslegeschrift No. 1082475.
Another known composition of this type, which is described in West German Auslegeschrift No. 1950560, contains hydrofluoric acid, a magnesium compound, at least one acid selected from nitric acid, phosphoric acid and sulfuric acid, which acids are in the form of free acid or of their salts and further containing a particular sulfonic acid. Although compositions of this type have been found to have advantages over other types of pickling and rust removing pastes, difficulties have sometimes been encountered in achieving the pasty consistency necessary for brush-on applications and also in providing the necessary quantity of pickling acid per unit surface of metal being treated. Attempts to overcome the problems of the consistency by the addition of thickeners has not been successful since such additions result in a further reduction in the content of the pickling acid that is present on the surface.
In the case of types of pickling pastes which contain a filler, such as barium sulfate, there is frequently at least a partial settling out of the filler during prolonged standing of the composition so that the paste has to be intensively stirred and homogenized before each application. Moreover, when such compositions contain volatile acids, such as nitric acid or hydrochloric acid, in addition to hydrofluoric acid, irritating and/or toxic vapors may be released from the liquid phase which forms as settling occurs, thus making handling of the composition, when the containers are opened, not only difficult but potentially dangerous. Finally, difficulties have also been encountered in rinsing such paste from the treated surface since the filler materials frequently collect in the pores of the metal being treated.
It is, therefore, an object of the present invention to provide a pickling and rust remover paste for the cleaning of metal surfaces, such as steel and/or aluminum, which is not subject to the aforementioned disadvantages of the prior art compositions.
A further object of the present invention is to provide an improved pickling and rust-remover paste which attains the desired paste consistency with relatively low additions of fillers; which can be easily removed from the metal surfaces to which it is applied; and which is also economical to produce.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.